Hydrogen sulfide is a major source of pollution of gas streams since it is liberated as a waste by-product in a number of chemical processes, such as sulfate or kraft paper pulp manufacture, viscose manufacture, sewage treatment, the production of organic sulfur compounds, as well as during petroleum refining and in the production of natural gas and combustible gases from coal, such as in coking operations. Hydrogen sulfide is also present in geothermal steam, which is captured for use in power generating plants.
To eliminate these polluting sulfur gases the art has developed several oxidationreduction (redox) processes that use an aqueous chelated metal catalyst solution for removing hydrogen sulfide from a gas stream. In those prior art processes a hydrogen sulfide-containing gas, known as "sour gas," is contacted with a chelated metal catalyst to effect absorption and subsequent oxidation of the hydrogen sulfide to elemental sulfur and concurrent reduction of the metal to a lower oxidation state. The catalyst solution is then regenerated for reuse by contacting it with an oxygen-containing gas to oxidize the metal back to a higher oxidation state. The elemental sulfur is continuously removed from the process as a solid product with high purity. Illustrative, but not exclusive, of these oxidation-reduction processes is the description contained in U.S. Pat. No. 4,622,212 (McManus et al.) and the references cited therein, all of which are incorporated herein by reference.
The principal operating problem encountered in such processes utilizing an aminopolycarboxylic acid chelating catalyst is the chemical degradation of the chelating agent, which requires addition of replacement chelating agent. The replacement of the chelating agent may represent a substantial operating cost, which will adversely affect the economic viability of the process.
McManus et al. teach that the degradation of an aminopolycarboxylic acid chelating agent occurs by severance or rupture of nitrogen-carbon bonds during the oxidative regeneration of the catalyst solution and that the degradation of the chelating agent can be retarded by incorporating certain stabilizing agents in the catalyst solution. One of the most effective stabilizing agents is the thiosulfate ion (S.sub.2 O.sub.3.sup.=).
Thiosulfate ions are made as a byproduct within (in situ) those oxidationreduction processing schemes having the oxidation of hydrogen sulfide and the regeneration of catalyst solution occurring within the same reaction vessel. However, in some oxidation-reduction processing schemes, such as a conventional oxidationreduction processing scheme, the oxidation of hydrogen sulfide occurs in a separate reaction vessel from the regeneration of catalyst solution; consequently, very little, if any, thiosulfate byproduct is produced in this process.
Up until now, the art has failed to come up with a method of producing thiosulfate within those processing schemes utilizing separate vessels for the oxidation and regeneration steps (i.e., in situ). Such a process represents an extremely economical method of reducing catalyst degradation, and consequently, operating costs. In addition, this invention allows for control over the amount of thiosulfate produced within the process. These and other advantages will become evident from the following more detailed description of the invention.